Light emitting diodes (LEDs) are increasingly being used for backlighting within display devices, such as liquid crystal display (LCD) televisions and monitors. Older technologies utilize CCFLs (cold cathode fluorescent lamps) for backlighting. The use of CCFLs has numerous drawbacks. For example, CCFLs contain mercury, which is a toxic material. CCFLs also require very large voltages to operate, such as between 1 kV to 5 kV. As a result, providing the driving function of CCFLs is difficult. The use of LEDs overcomes these drawbacks. LEDs do not contain mercury, and a single LED can operate at approximately 3.5 volts, which eases the system design requirements. Further, LEDs have a much longer operating life than CCFLs. The operating life of a CCFL is approximately 20,000 hours, where the operating life is defined as the period of time over which the brightness of the CCFL is maintained above a threshold level, such as one-half the original brightness. The comparable operating life of an LED is up to 50,000 hours.
In the context of backlighting applications, LEDs also suffer from drawbacks. Typically, an LCD television uses 200 or more LEDs, depending on the display screen size. In one configuration, the LEDs are connected in series. In this series configuration, the 200+ LEDs times 3.5 volts per LED still results in 700+V. In another configuration, the LEDs are divided into multiple strings, where each string includes a smaller subset of LEDs. For example, a 45-50 inch LCD television may have 4-10 LED stings, each string having 50-150 LEDs. LEDs being essentially p-n junction diodes, do not share current equally when connected in parallel. Instead, the LEDs are connected in series as a string, and thus each LED in the string shares the current. The strings then are driven individually. This string configuration enables operation at a lower voltage. The string configuration is the route that manufactures have chosen.
A design challenge of the string based configuration is that in order to provide uniform backlighting, each of the LED strings must be driven with equal current. This requires the use of regulated current sinks, or current sources. A lowest cost approach is to provide a common anode voltage to all LED strings and then regulate the current through each cathode by using linear current sinks built around a transistor, such as a FET or BJT. However, manufactured LEDs do not have identical voltage drops, and therefore the provided anode voltage must accommodate the LED string with the highest voltage drop. This results in increased power loss across the current sinks corresponding to those LED strings having lower voltage drops. Although individual LEDs or even LED strings can be binned according to similar voltage drops, this comes at an added cost to the LCD panel or television manufacturer. This still does not completely eliminate voltage drop differences in LEDs as all similar LEDs are still not identical and thus still results in some amount of power loss. A more efficient approach is to separately provide an independent anode voltage to each LED string and then regulate the current through the LED string using the linear current sink. The anode voltage applied to a specific LED string is determined according to the voltage drop of the specific LED string, not according to the LED string with the highest voltage drop. This results in optimized power loss across the current sinks. However, providing independent anode voltage to each LED string requires more circuitry and is more costly.
FIG. 1 illustrates a block schematic diagram of an exemplary conventional power circuit 10 used to power a plurality of LED strings. The power circuit 10 is an example of the second option described above where an independent anode voltage is provided to each LED string. A PFC boost regulator 12 receives and converts an AC input to a full-wave rectified sine wave current and regulates the current to 380 volts DC at the output. A regulated safety isolated DC/DC converter 14 converts the 380 volt signal to 24 volts, which is output to a boost regulator 16. The boost regulator 16 boosts the voltage from 24 volts to 100 volts. In some cases, the converter 14 can be configured to output 100 volts so as to eliminate the boost regulator 16. Multiple boost regulators 18-22 are connected in parallel to the boost regulator 16. There is one boost regulator for each LED string. For simplification, only one LED string 26 is shown in FIG. 1, which is coupled to the boost regulator 18. In implementation, an additional LED string is coupled for each additional boost regulator coupled in parallel to the boost regulator 18. The boost regulators 18-22 each boost the 100 volt input to 250 volts, thereby supplying the necessary voltage at point A for a current sink, such as a transistor 28, to be able to regulate the current for the corresponding LED string, in this case LED string 26.
The transistor 28 is coupled to the cathode of the LED string 26. A separate transistor is similarly coupled to each LED string. A linear current sink control and boost feedback circuit 24 is separately coupled to the cathode of each LED string, such as the LED string 26, and to each transistor, such as the transistor 28. The circuit 24 in conjunction with the current sinks function as current regulators for controlling the current provided to each LED string so that the brightness of each LED string is uniform. Power loss through each current sink, such as the transistor 28, is minimized by sensing the voltage at point A and providing appropriate feedback to the corresponding boost regulator, such as boost regulator 18, to regulate the voltage applied to the anode of each LED string, such as the anode of the LED string 26. There is a minimum compliance voltage that must be maintained at point A in order for the transistor 28 to function as an effective current sink and regulate the current through the LED string 26. The boost regulator 18 adjusts the output boost voltage applied to the anode of the LED string 26 such that the output voltage minus the voltage drop across the LED string 26 is equal to the minimum compliance voltage necessary at point A. A boost voltage that results in a voltage at point A greater than the minimum compliance voltage leads to power loss across the transistor 28. A boost voltage that results in a voltage at point A less than the minimum compliance voltage does not enable the transistor 28 to function as a current sink and therefore does not lead to proper current regulation through the LED string 26.
The minimum compliance voltage at point A can be set to any level, such as 1V, 10V, or 15V. The voltage level is set according to the transistor 28 and the actual voltage drop across the LED string 26. For example, the power circuit manufacturer receives the LEDs from a vendor, but the exact voltage drop for each LED string is not known because the specifications for each LED may vary. In order to accommodate this unknown voltage the power circuit manufacturer has two choices. One, the output of the boost regulator 18 can be fixed to a set output voltage level and the transistor 28 is allowed to absorb the extra voltage, which is the output voltage of the boost regulator 18 minus the voltage drop across the LED string 26. The transistor 28 burns this excess voltage as power by supplying the absorbed voltage as current to the transistor 28. This option presents an inefficient situation because in any given batch of LEDs, each LED string may have a voltage drop that is less than the highest possible voltage drop designed into the circuit. This may result in the total power burned through all the transistors coupled to the LED strings being upwards of a few watts. A way to circumvent this inefficiency is to measure the drain voltage of the transistor 28 (at point A) and supply that measurement as feedback to the boost regulator 18, which then adjusts the output boost voltage accordingly. This feedback method essentially regulates the voltage at point A to the minimum compliance voltage required for the transistor 28 to function as a current sink with the desired amount of current.
In the configuration of FIG. 1, the boost regulator 18 is conductive. The boost regulator 18 optimally adjusts the overhead voltage of the transistor 28 to the value that is necessary to maintain regulation of the current through the LED string 26, but not more than that. This is an optimum situation from an efficiency perspective. However, the power circuit 10 then requires one boost circuit (boost regulator) for each LED string plus a corresponding linear current sink controller circuit 24, one control loop coupled to one of the corresponding boost regulators 18-22. This makes for an expensive and complex system.
FIG. 2 illustrates a block schematic diagram of another exemplary conventional power circuit 40 used to power a plurality of LED strings. The power circuit 40 is an example of the first option described above, the lowest cost approach where a common anode voltage is provided to all LED strings. The power circuit 40 of FIG. 2 differs from the power circuit 10 of FIG. 1 in that the power circuit 40 has a single boost regulator 48 (instead of the multiple boost regulators 18-22) and a single boost feedback. The output of the boost regulator 48 is provided as a common boost voltage to the anode of each LED string. A current sink is coupled to each LED string, such as a transistor 54 coupled to a LED string 52. A linear current sink controller 50 includes separate control loops for each LED string and current sink pair. The separate control loops provide the single feedback to the boost regulator 48. The efficiency of the power circuit 40 of FIG. 2 is not as high as the efficiency of the power circuit 10 of FIG. 1. The reason is that the boost voltage is separately provided to each LED string in FIG. 1, and that each LED string current in FIG. 1 is being regulated by a corresponding linear current sink transistor, such as transistor 28, and a corresponding control circuit. Despite the simplicity and lower cost of the power circuit 40 in FIG. 2 compared to the power circuit 10 of FIG. 1, this benefit comes at the expense of lower efficiency because the single boost regulator output voltage must be regulated to level that accommodates the highest LED string voltage drop. In neither configuration, the power circuits do not fully eliminate power loss.